Laser Sintering is a process by which a three dimensional article may be formed in a layer-wise fashion by selectively projecting a laser beam having the desired energy onto a bed of resin particles. Prototype or production parts may be efficiently and economically produced by this process, which is often times referred to as Selective Laser Sintering (SLS, trademark of DTM Corporation, Austin, Tex.). This process has been described in U.S. Pat. Nos. 4,944,817; 5,516,697 and 5,382,308 to Bourell, et al.; 5,304,329 and 5,342,919 to Dickens, Jr. et al. and 5,385,780 to Lee.
Generally selective laser sintering technique and equipment use a laser which emits energy focused on a target area. In the target area, where the part is produced, is a powdered material which partially melts or softens under the energy emitted from the laser. The selective laser sintering equipment includes a means to deposits a smooth, level layer of the powdered material on to the target surface before the layer of powder is exposed to the laser energy. The laser energy emission is controlled and limited to a selected portion of the target area by a computer link to a CAD/CAM system which directs the laser to scan to form a "slice" of the part. After exposure of powdered material to form the first "slice" of the part, a second layer of powdered material is deposited into the target area. Again the laser scans the target area exposing only the portions of the target area as directed by the CAD/CAM program producing a second "slice" of the part. This process is repeated until the part is built up "slice by slice" to form the completed part.
It has been the practice that the laser energy to which the powder is exposed is just that amount that is sufficient to quickly form the part slice, and therefore, it has been necessary to heat the target environment so that the powdered resin is at or very close to its melting point before laser exposure. Thus, the thermal properties of the sinterable powder are important in assuring that there is a window of operation in the selective laser sintering process. That is, so that there is a minimal of polymer particle softening at some elevated temperature so that the powder can remain in the heated target environment with out the initiation of particle fusing until a later time at which a rapid, focused boost in thermal energy is supplied to the heated particles by the scanning laser beam.
There are several techniques known to provide this window of operation for the laser sinterable powdered materials. Generally these techniques rely on the mixing of powders from various materials, especially polymer materials having characteristics that provide a wide range in softening and melting characteristics, and selection of powder particle sizes that provide good packing properties. Widening the softening or melting range provides a powder that can be stored in the target area at a temperature that is close to the fusion point of the particles. The additional energy provided in the laser scan can quickly fuse the particles to form the "slice" of the cross section of the part without causing hot spots that may contribute to poor resolution of the part dimensions.
Providing a particle size distribution of a mix large and small particles or a bimodal particle size distribution not only provides small particles that will heat faster and, thus, fuse easily to form the slice of the part, but also provides smaller particles which may pack in between the interstitial spaces between the larger particles of the powder providing a means of densifying the fused part.
The window of operation has been widened by Buorell, for example, by the inclusion of a plurality of materials, either as a coated particle or as a mixture of particles. The materials in these mixtures have different softening temperatures, which may be widely separated from the other. The mix of such powdered materials produces a bulk powder that contains only a small amount of readily melting material. This same idea of differentiation of the softening or melting point of the polymer to provide a window of operation was illustrated in Dickens, et al. by not only including a bimodal particle size distribution, but also by using a semicrystalline polymer that has some amorphous character that possesses a softening point below the caking (fusion) temperature of the crystalline material.
Lee taught the use of a fairly high concentration of an anticaking material incorporated into the powdered polymer particle surface to preclude polymer particles from sticking together at an initial Tg (glass transition temperature) of the polymeric material. With the additional thermal energy provided by the laser exposure, the powdered polymer particles reached a second Tg, and the separate particles then melted or soften to the extent that they bonded to other such softened polymer particles forming a fused layer or "slice" of the part.
But in each of these techniques, the temperature in the target zone had to be relatively high, sometimes as much as 190.degree. C. The polymer particles, after laser exposure, became super heated and the parts formed had to be cooled for long periods of time before they could be removed from the bed of powdered resin. With the elevated temperatures required for maintaining the powdered polymer near the softening point, temperature control becomes difficult, and temperature variations are common. Temperature variations in the target area contributes to distortion, poor parts quality and to higher costs of operation.
In addition, parts made from prior art polymers had poor flexibility. The lower temperature softening and melting material acted like a glue to stick particles into ridged shapes. The high content of inflexible inorganic materials such as anticaking materials, or materials having very high melting points, functioned as reinforcement of the fused layer.
It is the object of the present invention to provide a polymer powder that can be easily sintered into flexible shapes at lower temperatures. The lower operating bed temperatures of the sinterable powders of the present invention provide greater temperature control, lower distortion of the objects formed therefrom as compared to the powders of the prior art.
It is also an object of the present invention that the part or article, after formation by sintering, be removed from the bed of sinterable powder particles immediately without having to wait for the article or part to cool slowly in the heated resin bed. A slow cool down step has been required to avoid stress and stress cracking that may develop when the part is cooled to room temperature too rapidly. Parts sintered from the powder of the present invention may be removed immediately from the bed of resin after formation or at any time during the formation process after a pass of the laser is complete. Removal from the resin bed immediately after a pass of the laser is complete will change the temperature of the part rapidly to room temperature, but does not cause stress to develop or stress cracking in parts formed from the powdered resin of the present invention. This property enhances the usefulness of the powdered resin of the present invention and avoids loss time between forming one set of parts and then forming a second set of parts, thus decreasing the cycle time of the laser sintering operation.
It is also an object of the present invention to provide a sinterable powdered resin that has a wide range of operation latitude. That is to provide a sinterable powdered resin that functions well over a wide range of temperatures, including room temperature, laser intensities and scan rates; and to provide a powder that is relatively insensitive to temperature variations in resin bed temperature if it is desired to sinter the powder from a bed that is maintained at some temperature above the ambient temperature.
It is also an object of the present invention to provide a powdered resin that is sinterable at low temperatures, but possesses physical properties similar to those of nylon and other powdered resins currently in use that require high resin bed operating temperatures.